Explanation
The error code INSUFFICIENT_ACCESS_OR_READONLY is caused by the cart being no longer in a valid Checkout State during checkout flow customizations. A cart is an object that represents a collection of products and charges that a customer intends to purchase in the storefront. A cart has a Checkout State field that indicates the current state of the checkout process for the cart. The Checkout State can have values such as Draft, InProgress, Completed, or Cancelled. A cart can only be modified or updated when it is in Draft or InProgress state. A cart cannot be modified or updated when it is in Completed or Cancelled state. If an attempt is made to modify or update a cart that is in Completed or Cancelled state, an error with the code INSUFFICIENT_ACCESS_OR_READONLY will be thrown. This error means that the user does not have permission to edit or delete a record because it is read-only or locked. The storefront user does not have access to the Cart Delivery Method object is not a cause of this error code, as it is not related to the cart checkout state or data modification. The Cart Delivery Method object is an object that stores information about the delivery method selected for a cart in the storefront. An error has occurred during the cart shipping charge integration is not a cause of this error code either, as it is not related to the cart checkout state or data modification. The cart shipping charge integration is an integration that calculates and applies shipping charges to a cart based on various factors such as delivery method, location, weight, volume, etc. The storefront user does not have access to custom fields on the Order Delivery Method object is not a cause of this error code either, as it is not related to the cart checkout state or data modification. The Order Delivery Method object is an object that stores information about the delivery method selected for an order summary in the storefront. Salesforce References: B2B Commerce Developer Guide: Cart Object, [B2B Commerce Developer Guide: Cart Delivery Method Object], [B2B Commerce Developer Guide: Order Delivery Method Object], [Salesforce Help: Common Error Messages]